1. Field of the Invention
The present invention relates to excitation systems for synchronous dynamoelectric machines, and more particularly to means for supplementing the excitation provided by a conventional brushless exciter for a synchronous generator.
2. Description of the Prior Art
Brushless excitation systems are now widely used for supplying direct current field excitation to synchronous dynamoelectric machines such as large alternating current generators. Such brushless excitation systems include an alternating current exciter having a stationary field structure and a rotating armature member. A rotating rectifier assembly is carried on a common shaft with the exciter armature and is connected thereto to provide a direct current output. The output of the rectifier is connected to the field winding of the main generator which also rotates with the exciter armature and rectifier. In this way, an excitation system is provided which requires no sliding contacts.
In conventional arrangements, the main exciter for a synchronous generator comprises an alternating generator having its armature mounted on the same shaft as the field winding of the synchronous machine and also having a stator field winding which must be energized by direct current to create a magnetic field so that a voltage will be induced in the rotating armature of the exciter. In well-known arrangements, the direct current excitation for the main exciter is provided by a pilot exciter having a permanent magnet rotor turned by the prime mover within an annular armature winding to produce excitation power for the main exciter. Means such as a rectifier circuit is ordinarily provided to convert the alternating current output of the pilot exciter to direct current for the main exciter field excitation.
This basic brushless excitation arrangement has proven to be entirely satisfactory in providing base excitation for rated voltage output and for providing forcing excitation for nominal speed of response levels. However, there is an increasing demand for synchronous generator systems having a speed of response, i.e., time rate of change of voltage output of 2.5-3.5. It has been the practice to increase the size of the permanent magnet in the pilot exciter to obtain sufficient forcing power to the main exciter field to achieve a faster speed of response. The speed of response of the synchronous generator is directly proportional to the strength of the magnetic field of the permanent magnet pilot generator which is in turn directly proportional to the mass of the permanent magnet rotor assembly. Thus, in order to achieve higher levels of speed of response, a larger permanent magnet pilot generator assembly must be provided. However, this may require a basic change in the mechanical arrangement of the brushless exciter system because of space limitations. The diameter of the permanent magnet generator rotor must be increased greatly to achieve a significant increase in the magnetic field output of the permanent magnet rotor. On some operating machines, a blower located on the permanent magnet hub of the pilot exciter is presently extended to its maximum permissible diameter consistent with permissible dynamic loading. Furthermore, even if no blower were required, a larger permanent magnet assembly would require larger bearings and a stronger supporting structure. Thus, there exists a need for means operable to supplement the excitation provided by the conventional brushless exciter to provide faster speed of response without increasing the size of the permanent magnet assembly of the pilot exciter.